Additively Manufactured Sound-Absorbing Porous Structures for Airfoil Trailing-Edge Noise Control

This paper presents the results of experimental investigations on the sound absorption of two porous structures and the far-field noise performance of an airfoil employing sound-absorbing porous structures at the trailing edge. The porous structure consists of a microperforated housing with an air gap (PA structure) or an acoustic foam insert (PF structure). Sound absorption coefficients have been characterized using an impedance tube. Compared with a commercial acoustic foam, PF structures have shown a consistently higher (up to 0.5 higher) sound absorption coefficient over 0.5-6.4 kHz. Moreover, the frequency characteristics of PA and PF structures are predictable, which allows their geometric optimization to suit different applications. Far-field noise of airfoils has been measured using a 64-channel microphone array at various flow conditions. The sound-absorbing microtube structure is found to reduce the trailing-edge (TE) noise by up to approximately 12 dB at lower frequencies (1.5-6 kHz), while increasing high-frequency (over 6 kHz) noise levels due to the roughness elements of porous geometries and unsteady flow permeation. The peak frequency of noise reductions is found to not scale with the flow speed, but remains within a narrow frequency band at different flow conditions (U-infinity=30-50 m/s, alpha(g)=0 degrees-10 degrees). This is consistent with the narrow-band feature of microtube structures' sound absorption, which indicates the contribution of sound absorption to TE noise reduction. The PF structure, due to its broadband sound absorption and tunable frequency characteristics, is considered a promising device for trailing-edge noise control. (C) 2021 American Society of Civil Engineers.